It may do nothing, but it has been an valid argument historically, and it's now embedded in our v1.0 API. We can't remove it without breaking any code that is using it. Marking someday/maybe, because this should be a v2.0 change.

lookup_overrides appears to be undocumented - I can't find any mention in the 1.0, 1.2, and dev docs.
Which code would rely on an undocumented argument that does nothing (and apparently hasn't done anything useful for quite some time)?